This invention is used for testing optical fibers in communication systems. This invention further relates to a non-destructive test method for simple and yet accurate measurement of optical loss characteristics of an optical fiber by coupling light by means of an optical fiber at a location other than the ends of the fiber. The invention also includes an apparatus for implementing the method therefor.
There are two representative methods for measuring optical loss characteristics in the prior art. The cutback method calculates optical losses by comparing the power received on the input side of a test optical fiber with the power received at the output fiber end face. The backscatter method obtains optical loss characteristics by measuring backscatter of an optical signal which is excited within the fiber, scattered by Rayleigh scattering and propagated in the direction of the input fiber end.
The cutback method comprises, as shown in FIG. 1, the steps of directing an optical signal from an optical source 2 into one of the end faces of an exciting fiber 1. A test optical fiber 3 is connected to the other end of the exciting fiber at point A and the light is coupled thereto. The optical signal at the other end face is detected with an optical detector 4. Then the optical fiber is cut at a point B which is several meters apart from the connection point A of the exciting fiber. The optical signal at the point B is detected. The optical power received at both points is compared in order to calculate optical loss.
In this method, the test optical fiber is cut in order to precisely measure the optical power coupled to the test optical fiber 3 and to exclude from consideration the irregular loss at the connection point A. Although the measurement precision is high, the method involves complicated work in cutting and connecting the exciting optical fiber 1 with the test optical fiber 3. It is further defective in that the cost for tests at manufacturing plants is high because the optical fiber must be cut by several meters or a length equal to the distance from the point A to the point B every time an optical fiber is tested. This leads to higher product prices.
The backscatter method, as shown in FIG. 2, comprises the steps of directly inputting an optical signal into an end face of a test optical fiber 3 from an optical source 2 using a high output pulse semiconducter laser. The backscattered light, which is light which is generated in the test optical fiber 3 propagated in the direction of the input fiber end, is separated with a directional coupler 6 and measured with an optical detector 4. This method does not involve complex work nor does it destroy test optical fibers. However, it additionally requires a directional coupler 6 and other optical components for separating the input light from the backscattered light. It further involves more sophisticated techniques and requires optical and electric components because, in this method, a weak backscattered light is received by the optical detector 4 using APD. The detected backscattered light is processed with an averager 7 in order to separate noise from signal and displayed by a display 8. This inevitably results in a higher price of the testing procedure.
An object of this invention is to provide a method and apparatus for measuring optical loss characteristics which is simply constructed and very precise and which does not require destruction of test optical fibers.